A woman aged 24 years presented with ataxia, visual obscurations, and headaches over 4 weeks. She had no significant past medical history, and was taking no medication. MR imaging revealed an enhancing mass in the fourth ventricle, which was removed at neurosurgery. Macroscopic examination revealed soft gray tissue measuring 10x5x5mm and several fragments of beige / gray tissue measuring 5x5x4mm, all of which were submitted for histopathological examination.

MICROSCOPIC DESCRIPTION:

Microscopy showed a tumour consisting of sheets of cells interrupted by a network of capillaries and one focus of necrosis. Additional architectural features were scattered rosettes and canalicular structures (Arrow - Fig. 1a), but there were no convincing pseudorosettes. Cytologically, there was a striking dimorphism manifesting either as groups of uniform cells with round or oval nuclei and indistinct cytoplasmic borders (Fig. 1a), or as areas where giant tumour cells exhibited gross pleomorphism and well-defined cytoplasmic borders (Figs. 1b & 1c). Both areas were occasionally crossed by fascicles of spindle-shaped cells with fibrillary processes, around which were scattered Rosenthal fibers (Fig. 1d). Reticulin was sparse.

Immunohistochemistry demonstrated that most tumour cells were GFAP-positive (Figure 2). No reactivity was detected with neuronal antibodies, anti-cytokeratin antibodies, or antibodies to epithelial membrane antigen. The maximum Ki-67 labeling index was less than 10%.

Ultrastructural examination revealed microvilli and cilia, as well as the presence of complex intercellular zipper-like junctions (Figure 3).